This invention relates to a damper, and more particularly to a method for manufacturing a bottom valve seat for use in a damper, which method includes the steps of: punching out a circular blank from a steel plate having a given thickness by a punching press to prepare a circular blank having given dimensions; drawing the blank into a cup-shaped work having a circumferential flange; further pressing the bottom portion of the cup-shaped work downwards to provide a tapered valve seat surface on the aforesaid bottom portion; and punching a through-hole in the central portion of the bottom portion of the cup-shaped work; thereby providing, according to a press-forming technique, a bottom valve seat provided with a valve seat surface and a through-hole.
FIGS. 1, 2 and 3 show a hydraulic damper, 1 being a cylinder, 2 a piston reciprocable within the cylinder, 3 a slidable piston rod, 4 a bottom valve seat, 5 an outer cylinder surrounding the peripheral surface of the cylinder, 6, 7 upper and lower end covers secured to the outer cylinder, 8 a rod guide adapted to guide the piston rod 3, and 9 a seal packing which seals a through-hole in the upper end cover 6.
When the piston 2 is slidingly lowered within the cylinder 1 so as to effect the stroke of a piston in a hydraulic damper, then oil in lower chamber A under the piston 2 passes through a through-hole 10a in a valve retainer 10, while urging the circumferential edge portion of the valve retainer 10 and the circumferential edge portion of annular double-sheet valve 11 against a valve seat surface 4a of the bottom valve seat 4, and then into a central opening 4b provided in the bottom valve seat 4 as shown by an arrow 20, while deflecting the inner circumferential edge portion of the valve 11. Thereafter, oil passes through oil passages 7a defined by several projections formed on the top surface of the lower end cover 7, and then into a reservoir chamber C as shown by an arrow 21.
Meanwhile, when the piston 2 is lowered, the annular double-sheet valve 24 provided in the piston 2 closes a through-hole 25a in the valve retainer 25, and slides together with the retainer 25 against the force of a spring, so that oil in the chamber A directly flows into the chamber B, without resistance.
When the piston 2 slides upwards within the cylinder 1 so as to extend the stroke of the piston, then oil in the upper chamber B above the piston 2 flows by way of a valve 24 provided in the piston 2, into the chamber A, producing hydraulic resistance, while oil in reservoir chamber C passes through the oil passages 7a provided in the lower end cover 7, as clearly shown by arrows 26, 28 in FIG. 3, and then through a hole 4b in the valve seat 4, thereby pushing the valve 11 and valve retainer 10 upwards against the force of a spring 27. Then, oil passes in between the valve seat surface 4a of the bottom valve seat 4 and the valve 11, then through a gap between the valve 11, valve retainer 10 and the valve guide 12 into the chamber A. In this manner, a difference in timing of oil flows in response to the reciprocating movements of the piston 2 provides a damping action.
With a hydraulic damper of this type, the valve seat 4 plays an important role, so that even a partial failure thereof would lead to a vital damage of the damper itself. In addition, the valve seat 4 is extremely complicated in construction.
For this reason, many attempts have been proposed hitherto so as to provide a high strength valve seat, which can accommodate itself to any complicated shape, and which can be mass produced.
To date, it has been found most suitable to manufacture a valve seat according to a sintering process. FIG. 4 shows an ordinary type valve seat which has been manufactured according to a sintering process.
At the present time, it is a widely accepted practice for the manufacture of a bottom valve seat to use an iron-base sintered material produced by compressing iron powder into a shape and sintering same. This method has been adopted from viewpoints of strength, limitation on the type of materials, and adaptability to forming. However, this sintering process poses another problem. For instance, a sintering process required a complicated step for correcting dimensions of a product by means of a correcting metal die for achieving a desired dimensional accuracy. This is because according to the sintering process, iron powder is placed in a cavity defined by a combination of a several metal dies and compressed to a given density to be shaped. Then it is taken out from the metal dies, and placed in a furnace for heating at 1100.degree. to 1200.degree. C for about 2 to 3 hours for sintering. Then, the dimensions of the workpiece are corrected by using a dimension-correcting metal die. In addition, the iron-base sintered material is vulnerable to impact, so that special care should be taken for manufacture and transportation of the products. Moreover, the iron-base sintered products suffer from an increased error in dimension along its length, as compared with an error in dimension in the diametrical direction. In addition to a complicated process, operations incident to processing of iron powder require considerable expense, thus resulting in a high manufacturing cost.